Infrared or thermal imaging systems typically use a plurality of thermal sensors to detect infrared light radiating from a scene. Such systems are often used to detect objects (e.g. people, vehicles, and planes) and situations (e.g. fires and overheating machines), and usually process sensor signals to produce an image capable of being visualized by the human eye.
The basic components of a thermal imaging system typically include optics for collecting and focusing infrared radiation from a scene, an infrared detector having a plurality of thermal sensors for converting detected radiation to electrical signals, and electronics for amplifying and processing electrical signals for control or visualization purposes or for storage in an appropriate medium. Each sensor can represent a pixel of video displaying the thermal image.
Thermal detectors are typically comprised of a plurality of thermal sensors, creating a sensor array. The sensor array is usually electrically coupled to an integrated circuit substrate through an array of contacts. To maximize thermal isolation of the sensor array from the integrated circuit, a thermal isolation structure is often disposed between the sensor array and the integrated circuit.
Thermal sensors are sometimes comprised of a pyroelectric capacitor, wherein each pyroelectric capacitor is comprised of a thermally sensitive dielectric material between two electrodes. Infrared radiation striking the pyroelectric capacitor heats the pyroelectric material which exhibits a state of electrical polarization or change in dielectric constant in response to the incident infrared radiation. Accordingly, the electrodes operate to measure the charge generated by the pyroelectric material in response to temperature changes. The charges detected in each sensor may be amplified and processed to form a visual display.
A method of absorbing and trapping infrared radiation is necessary to allow sensors to detect incident thermal energy. Generally a cavity, with a thickness 1/4 that of the wavelength of infrared radiation sought to be trapped, is established to trap the incident infrared radiation. One approach establishes a 1/4 wavelength cavity between a sensor and the integrated circuit substrate. This method requires that both sensor electrodes be composed of semi-transparent metal. However, semi-transparent metal is difficult to fabricate because the thickness of the metal must be very thin and high temperatures are required to process the layer, challenging the maximum temperature of the integrated circuit.
Another method of absorbing and trapping infrared radiation involves establishing a 1/4 wavelength cavity in the sensor itself by utilizing an infrared semitransparent top electrode and infrared opaque bottom electrode. However, this method requires thicker pyroelectric material between the electrodes, preventing formation of thinner pixels. Accordingly, there exists a need in the art for an improved thermal sensor and method to eliminate difficult fabrication steps and allow for the development of a thinner thermal sensor.